Only be thou strong and very courageous, that thou mayest observe to do according to all the law, which Moses my servant commanded thee: turn not from it to the right hand or to the left, that thou mayest prosper whithersoever thou goest. (Joshua 1:7)
Yes, the Lord will be with us in our holy war, but He demands of us that we strictly follow His rules, Our victories will very much depend upon our obeying Him with all our heart, throwing strength and courage into the actions of our faith. If we are halfhearted we cannot expect more than half a blessing.
We must obey the Lord with care and thoughtfulness. “Observe to do” is the phrase used, and it is full of meaning. This is referred to every part of the divine will; we must obey with universal readiness. Our rule of conduct is “according to all the law.” We may not pick and choose, but we must take the Lord’s commands as they come, one and all. In all this we must go on with exactness and constancy Ours is to be a straightforward course which bends neither to the right nor to the left. We are not to err by being more rigid than the law, nor turn out of levity to a more See and easy way. With such obedience there will come spiritual prosperity. O Lord, help us to see if it be not even so! We shall not test Thy promise in vain.
C.H. Spurgeon, Faith’s Checkbook, Faith’s Checkbook